The conflict in Syria is regional, ancient and, when all is said and done, impervious to outside, top-down reordering and interventions.
Doug Bandow in Chronicles:

Donald Trump campaigned on an “America First” foreign policy. But he hasn’t been immune to the vapors of the Swamp. Not even three months after his inauguration, administration officials were praising NATO; affirming commitments to Japan and South Korea; discussing troop surges for Afghanistan; talking about permanently stationing forces in Iraq, increasing aid for Saudi Arabia’s war against Yemen, and effecting regime change in Syria. It was as if Hillary Clinton occupied the President’s body.
Trump’s flip-flop on Syria was particularly shocking. Before the dawn of the Neoconservative Age no sane American would have suggested intervening in the horror that this ancient land has become.

… The Syrian conflict quickly turned into a proxy war: The U.S., Turkey, Saudi Arabia, and the other Gulf states pushed for Assad’s ouster, while Iran (including Afghan and Iraqi militias under Tehran’s command), Lebanon’s Hezbollah, and Russia supported the Damascus government. Turks and Saudis were pleased to work with the Islamic State and Jabhat al-Nusra (the Support Front), which was the local Al Qaeda affiliate; meanwhile, Washington actively battled ISIS while tolerating al-Nusra. True to form, Ankara viewed Kurdish militias (allied with Washington) as the most dangerous faction and focused its malign military attention on them….